Yes and no. In the strictest biological sense babies–whether preborn or not–cannot be parasites because, first, they are of the same species as their parents. Tapeworms or hookworms or leeches can all be parasites as they are not of the same species as the animals they infest. In that literal biological sense, human fetuses are not parasites. Second, no textbook in embryology or fetology categorizes the human fetus as a parasitic organism. Third, parasitic organisms are categorized as such regardless of their stage of development, but it would be an overreach to universally label as “parasites” all humans beings throughout their lifespan. If they start as parasites in the womb, they’d need to be parasites even out of the womb and on to adulthood. Fourth, it is presumed that young members of any given species are liable to be deeply dependent on their parents or community. Fifth, the mammalian mother, in particular, is physically and psychologically built to nurture a gestating child which shares half her DNA. The relation, then, is intrinsic being within the family and, for what it’s worth, “natural.” The human race could not survive without this relation, meanwhile no such hosts in the rest of the animal kingdom need their respective parasites to this degree. Sixth, with the interpersonal dynamic of mother to child, that relationship is more symbiotic than parasitic. Parasites, on the other hand exploit incidental features of their hosts with no interpersonal relation while typically harming the host. Such a host might have warmth and nutrition for a parasite, but it’s a difficult case to parallel that with the mother’s fantastically fine tune environment for the developing child inside her. Seventh, the child-in-utero is not necessarily or normally a mortal threat to the mother, meanwhile the normal or “natural” disposition of parasites means harm and often mortal danger to the host. Some pregnancies incur medical complications putting the mothers health or life at risk, but–unlike parasitic relations–these dangerous pregnancies are not the normal, necessary, or intrinsic relation of child to mother.

However, in a less literal sense, babies–whether preborn or not–parallel parasites in that they feed off of the nutrients fed to them through their mothers. They are utterly dependent on their mothers in utero, and ex utero they are utterly dependent on their mothers or whoever else cares for them.

Stretching the analogy further, human beings stay pretty parasitic through much of their life. Parents of teenagers can attest to how their children are not fully functional adults without their insight, care, and provision. Handicapped adults or senior adults can also be deeply dependent on others, even harming their respective “hosts” by needing more of them than can be provided.

So the case can be made that people at most any stage of life are capable of being parasitic, even if, strictly and biologically speaking they are not parasites.

Now where does this exercise get us? It is a well-known aim of pro-life advocates to emphasize all the dignifying terms for the human fetus–baby, child, human, human being, boy/girl, person, etc.Meanwhile, pro-choice advocates are known to emphasize demeaning terms for the human fetus–such as fetus, tumor, clump of cells, or parasite.* Presumably, if one grants one of these dignifying or demeaning titles for the human fetus, one has granted some of its correlate ideas. Parasites are weed-like animals, unwanted, unsightly, something to be rid of. However, insults do nothing to change the biological status of a human being–and biologically the human fetus is literally a human being. A parasitic human being is still a human being with all the innate rights therein, whether it’s your brother-in-law, your great grandfather, or the child in your wife’s womb.

The “parasite” affiliation runs the risk of a “loaded word” fallacy, proving nothing in directly logical terms but instead slandering the preborn child with connotations of “weed,” “unwanted,” “harmful,” and “deadly.” To justify killing a human being, something more than “parasite” would need to be established, such as “deadly assailant” or “murderer” or “rapist.” That case can be made if the child represents an distinct medical threat to the mother’s life, such as with tubal pregnancies. The baby is instrumentally threatening the mothers life, and so it would not be a loaded word but an apt description to call that preborn child–instrumentally deadly. That is an unfortunate but real scenario, yet when it occurs, that child’s presence threatens the life of the mother, and might be justifiably aborted on the grounds of self-defense. Those sorts of cases are widely admitted justifications for killing a human being, but apart from tubal pregnancies (and similar cases) the death penalty just doesn’t apply. The child is not a “Murderer” or a “rapist” or a “traitor.” He (or she) does not deserve the death penalty. Those terms don’t apply. One might argue that the child is a “trespasser” and it’s legal in many states to shoot and kill a trespasser. However, its not legal to a kill a person whom you’ve invited onto your property. And frankly, sex has been nature’s way of inviting a baby into the womb since time immemorial. The baby is not a trespasser, it’s an invited guest. Even with contraceptive practices, one is still inviting a child into one’s “home” despite the fencing and locked door. Sometimes that child still gets in the house. It’s not a trespasser though. It seems, then, that the apt words don’t apply and the word “parasite” is a loaded word.

The “parasite” affiliation is also a non-sequitur. It “does not follow” logically into a pro-choice position especially since all human beings go through a stage where they are similar to parasites. If parasites should be destroyed, then–following the logic to it’s absurd ends–it seems all human beings should be destroyed. Even if one admits that parasites are also unwanted, and many children in utero are wanted, that does not change the fact that lots of brothers-in-law sleeping on the couch are “unwanted” guests. Why are they allowed to live there? Because it’s one’s family duty, or it’s the charitable thing to do, or he could really use some help right now after his divorce. In short, overriding family responsibilities–at least in that case–contradict the notion that unwanted dependents can rightly be expelled from one’s domain eventuating their death.

Obviously, the pro-choice position does not advocate that all human parasites deserve death, nor that most parasitic people deserve to be kicked out of whatever residence they occupy. Nevertheless, those would be logical outcomes if the parasite argument were employed without the gracious gap of its non-sequitur status. By not bridging that gap from the premise to conclusion, the pro-choice advocate can remain aghast at the conclusions just drawn from the parasite affiliation. That means, then, that the parasite affiliation must be a non-sequitur or an ad absurdum.

Lastly, it can be admitted that children, in utero or ex utero, are very dependent human beings, desperately needing the their parents. But just as puppies and kittens need extra love and care, so do baby humans. Their dependence is just cause for loving them tenderly and caring for them selflessly. Their dependence hardly excuses neglecting and killing them, as implied by this pro-choice lingo.*Rarely is it termed “human fetus” though that distinction is technically correct since horses, pigs, and whales, etc., can have fetuses, but only humans can have human fetuses.

Eyeon, “By definition, parasites are those animals which occupy the last niche [in the list of terrestrial, aquatic, and biotic], i.e. live in or on another species, their host. Parasitism is a form of symbiosis, an intimate relationship between two different species.” (http://parasite.org.au/para-site/introduction/introduction-essay.html). Another medical source from UPENN agrees pointing out how the parasitic relation is between two species. Other sources, offering less detail, have been known to point out their harmful relation to their host (see “parasite” and “parasitism” in this health resource).

The OED is a good general dictionary, but when it comes to medical, legal or philosophical definitions, the respective sort of dictionary is more appropriate.